Handle 3rd-party deps for PhidgetSpatial device

[PeterBowman]

As a follow-up to #163, there should be a clean way to enable the PhidgetSpatial device and let CMake tell us whether required deps are available or not (just don't ignore that and make it crash on compile time). Necessary headers and libs are currently located in extern/phidget/. Ideas:

• If enabled, instruct CMake to unzip the TAR file and fetch its contents into the build process. Warn the user and disable the device otherwise.
• Write a FindSpatial.cmake module and proceed as usual (keep in mind: Decouple external build configuration (Travis CI/YCM) from local settings questions-and-answers#18).

[PeterBowman]

FIXME (cmake-opts-on branch):

yarp-devices/libraries/YarpPlugins/PhidgetSpatial/CMakeLists.txt

Line 8 in 81df049

DEFAULT OFF) # FIXME: resolve #165 and set to ON

[jgvictores]

• Write a FindSpatial.cmake module and proceed as usual (keep in mind: Decouple external build configuration (Travis CI/YCM) from local settings questions-and-answers#18).

+1 to this option. Additionally, seems like option that can provide better support for different OS.

[jgvictores]

Related: #167

[PeterBowman]

Done at 37f39f8 (find module + docs for PhidgetSpatial device + build coverage on Travis).